The present invention relates to a method of and an apparatus for controlling the wall thickness in an extruded synthetic-resin blank. More particularly this invention concerns the production of such a blank which is thereafter to be blow molded into a container such as a bottle.
In the production of hollow bodies such as containers, tubes, and the like it is standard practice to transform a solid and plastic synthetic-resin strand first into a tubular and plastic synthetic-resin strand. Thereafter this tubular strand is formed in a mold or similar equipment into the desired article.
It is almost always necessary that the desired article has a uniform wall thickness, that is that it be of substantially the same wall thickness throughout.
Typically the solid strand is fed to a shaping nozzle having a central mandrel which transforms the solid strand into a tubular strand that passes through an annular passage which may contain several large-volume compartments to an annular outlet. Pistons may be provided in these compartments or the synthetic-resin material may simply be forced through the passage continuously so that a tubular synthetic-resin blank issues from the annular outlet of the shaping nozzle. The pin or mandrel that transforms the solid strand into a tubular strand is typically constituted as the core or central part of the nozzle and is supported on the outer part of the nozzle by means of struts or webs. These last-mentioned struts or webs must inherently extend radially through the passage along which the synthetic-resin material flows so that the flow must pass around these struts or webs. After flowing around such formations the material reunites. To this end the core may be made of oppositely tapered shape so as to maintain a uniform flow cross-section throughout the passage. It is also possible to angularly offset axially spaced struts or webs in order to minimize the interruption of flow for the tubular strand.
It has also been suggested in arrangements wherein the solid strand is introduced radially into the shaping nozzle to subdivide this strand into a pair of branches which each are eventually fed at the outlet side of the nozzle to a respective semi-cylindrical passage. To this end the distribution chamber inside the shaping nozzle is heart-shaped and the points of such heart-shaped chambers overlap each other. Various other formations are used to insure uniform flow and it is also known to profile the core of such a mold. Such arrangements can be seen in U.S. Pat. Nos. 3,114,932; 3,217,360 and 3,309,443 whose entire disclosures are herewith incorporated by reference.
Further known arrangements can be seen in Canadian Pat. No. 788,896, French Pat. No. 1,279,158, German Pat. Nos. 1,236,173 and 1,704,850 as well as in German Utility Model No. 1,984,772.
All such arrangements have nonetheless shown themselves not able to produce finish products after blow molding having the exactly required wall thicknesses. In particular in the production of bottles or the like of synthetic-resin material the desired thickness at and adjacent the base and neck of the bottle is almost impossible to obtain with the known methods. Furthermore when the composition of the material being molded changes it is normally necessary to replace the entire nozzle assembly in order to insure proper molding of the new material.
Furthermore due to the flow characteristics of the synthetic-resin material it is frequently necessary to decenter the core of the mold. The result of this is that the plastic synthetic-resin material flows most rapidly in those regions where it is the hottest or where the pressure drop is the greatest. It is necessary to reduce the mold gap or passage width at exactly this location in order to obtain uniform flow throughout the shaping nozzle. When this is done the blank produced by such a mold is of irregular thickness and hottest in the thinnest region, so that it becomes very difficult to form many types of articles with such a blank. When such a virtually randomly irregular blank is used to produce a container or the like the uncontrollable irregularities in the blank are often multiplied during the blow molding.